Filter
Top Products
182 CHAPTER 8: STP OPERATION
Designating Switches
and Ports
A designated switch is a switch in charge of forwarding packets to the local switch
by a port called the designated port. For a LAN, the designated switch is a switch
that forwards packets to the network segment by the designated port.
As illustrated in Figure 50, Switch A forwards data to Switch B through Ethernet
port 1/0/1. So to Switch B, the designated switch is Switch A and the designated
port is Ethernet 1/0/1 of Switch A. Also, Switch B and Switch C are connected to
the LAN and Switch B forwards packets to the LAN. So the designated switch of
the LAN is Switch B and the designated port is Ethernet 1/0/4 of Switch B.
Figure 50 Designated Switch and Designated Port
Calculating the STP
Algorithm
The following example illustrates the calculation process of STP.
The Figure 51 illustrates the network.
Figure 51 Switch 7750 Networking
Only the first four parts of the configuration BPDU are given in the example. They
are root ID (expressed as Ethernet switch priority), path cost to the root,
designated switch ID (expressed as Ethernet switch priority) and the designated
port ID (expressed as the port number). As illustrated in the figure above, the
priorities of Switch A, B and C are 0, 1, and 2 and the path costs of their links are
5, 10, and 4.
Switch A
Switch C
Switch B
LAN
E1/0/1
E1/0/7
E1/0/4
E1/0/2
E1/0/5
E1/0/1
Switch A
with priority 0
S
witch B
w
ith priority 1
Switch C
with priority
2
E1/0/5
E1/0/2
E1/0/1
E1/0/1
E1/0/7
E1/0/4
4
5
10